Kerr cells and Pockels cells are particular types of electro-optic modulators (EOMs) that can modulate the polarization of light incident on them. In an EOM, an electric field is applied to a material that changes properties under the influence of the electric field. The EOM's change in properties modifies the phase of light transmitted therethrough. Pockels cells are based on the Pockels effect, in which a material's refractive index changes linearly with applied electric field. Kerr cells are based on the Kerr effect, in which a material's refractive index varies quadratically with the applied electric field. For certain materials and certain orientations of the applied electric field, the Pockels effect creates an anisotropy in the refractive index of the material. Such materials and fields may be used to create a Pockels cell, in which the induced anisotropy changes the polarization state of light transmitted therethrough linearly as a function of applied voltage. EOMs such as Pockels cells may be placed between crossed polarizers to modulate the intensity of light. The temporal response of a Pockels cell may in some circumstances be less than 1 nanosecond, enabling its use as a fast optical shutter.
Although widely used for laser applications, Pockels cells traditionally have been viewed as having significant limitations, rendering such devices unsuitable for optical processing in other types of applications. Pockels cell materials have birefringence (different values of the refractive index for light polarized along different axes of the crystal structure), which restricts the angular acceptance of incoming light to the cell. Some known Pockels cells may only effectively modulate incident light deviating by less than few degrees from the surface normal of the Pockels cell, significantly limiting their use in such applications. For example, the paper “Extending the field of view of KD*P electrooptic modulators,” by Edward West, Applied Optics, Vol. 17 No. 18, pp. 3010-3013, September 1978 (“West paper”), discusses using compensation techniques to achieve larger acceptance angles for Pockels cells. However, the paper also describes how the compensation techniques have a small impact on the acceptance angle when voltage is applied to the Pockels cell during operation. Thus, the compensation techniques of the West paper are not entirely useful for wide acceptance angle applications, such as imaging, where incident light may hit a Pockels cell at larger range of incident angles.